A well-known method of determining the distortion of binary signals is through the use of a so-called eye pattern, e.g. as described in the book Data Transmission by Davey Bennett, McGray-Hill Book Co., 1965, pp. 118-121, Chapter 7--7. By this method, visual observation of an oblong trace on an oscilloscope screen gives an estimate of the distortion introduced along the transmission channel, the ratio of the internal to the external height of the pattern being a measure of signal quality. Other conventional techniques include the use of the so-called PAR meter described in an article by John H. Fennick, IEEE Transactions on Communication Technology, Vol. Com-18, No. 1, February 1970; that system requires the transmission of a special test pulse. Other prior-art systems rely on certain predetermined characteristics of a transmitted pulse sequence, such as the presence of parity bits and other redundancies. Reference in this connection may be made to CCIR Report 613, entitled "Bit Error Performance Measurements for Digital Radio Relay Systems", CCIR Vol. 9 (UIT), Geneva 1975.
Though the last-mentioned type of system enables the continuous surveillance of a transmission path by automatic means, it requires costly multiplexing and demultiplexing equipment for the insertion and recovery of the redundancy bits. Moreover, the detection of a statistically sufficient number of transmission errors in such a system generally requires considerable time, especially if a high degree of sensitivity is desired, so that corrective adjustments may not keep pace with changes in the transmission-channel characteristics. Finally, modern radio links used for the transmission of binary signals generally perform satisfactorily for extended periods and are only intermittently subject to substantial distortion. The error rate, however, varies by several orders of magnitude in response to changes in the signal-to-noise ratio by only a few decibels. Thus, the average error rate is not a dependable indicator of the sharp signal degradations occurring during less than 10% of total transmission time.
In an article by D. J. Gooding entitled "Performance Monitor Techniques for Digital Receivers, based on the Extrapolation of Error Rate", published in IEEE Transactions Com-16, pp. 380-387, June 1968, there is described a method of determining the transmission-error rate by feeding the incoming binary signals, demodulated from their carrier, to a set of ancillary decision networks in parallel with a main decision network delivering these signals to a load. These ancillary networks have different thresholds for the evaluation of signal amplitudes, the absolute values of the thresholds of the ancillary networks exceeding that of the threshold of the main decision network whereby one or more of these ancillary networks will register deviations from the output of the main network, according to the degree of degradation. These deviations, termed "pseudo-errors", occur more frequently than actual errors (i.e. inconsistencies between the transmitted signal and the signal reconstituted by the main decision network) and can therefore be counted more rapidly to provide an evaluation of the channel characteristics in a shorter time. An estimate of the actual transmission-error rate is obtainable from an extrapolation of the readings of the several pseudo-error counters.